FO OF A WDB ACR ACFT INADVERTENTLY SET MILLIBARS INSTEAD OF INCHES IN THE ALTIMETER RESULTING IN OVERSHOOTING DSCNT ALT.

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RECEIVED ATIS ABOUT 200 MI OUT AND IT WAS COMING IN KIND OF BROKEN. MISTOOK 991 MILLIBARS FOR 2991 INCHES FOR THE ALTIMETER SETTING. SO WHEN APCH CTL ALSO SAID 991 FOR THE ALTIMETER SETTING WE WERE LOCKED INTO THINKING 2991 INCHES (A COMMON ALTIMETER SETTING IN UNITED STATES). WE WERE CLRED TO DSND TO 3000 FT AND WE DSNDED TO 2500 FT AND THEN IMMEDIATELY PULLED BACK UP TO 3000 FT AS WE NOTICED OUR MISTAKE. WE WERE IN NO CONFLICT WITH OTHER ACFT AND WE WERE 2500 FT ABOVE THE GND. IT WOULD BE HELPFUL IF INTL CTLRS OR ATIS WOULD GIVE ALTIMETER SETTINGS IN BOTH MILLIBARS AND INCHES TO FOREIGN ACFT! SUPPLEMENTAL INFORMATION FROM ACN 253129: CREW CONSISTED OF A BRAND NEW CAPT ON FIRST TRIP; A COPLT WHO HADN'T FLOWN IN 5 WKS; AND MYSELF AS RELIEF PLT. I USUALLY FLY WITH VERY EXPERIENCED CREW MEMBERS AND DID NOT ADEQUATELY BACK UP MY FELLOW PLTS. APCH CTL QUESTIONED OUR ALT. WE IMMEDIATELY DISCOVERED OUR MISTAKE AND RETURNED TO THE CORRECT ALT. THIS EPISODE WAS CAUSED BY FATIGUE; INEXPERIENCE; AND UNWARRANTED ASSUMPTIONS. THE PROB WAS ABLE TO DEVELOP; I FEEL; BECAUSE OF NO STANDARDIZED WAY OF RPTING ALTIMETER SETTINGS IN EUROPE. SOME ARPTS RPT ONLY MILLIBARS AND SOME (TO BE HELPFUL) RPT BOTH MILLIBARS AND INCHES OF MERCURY. THEN; SOME RPT QNH; ABC; XYZ AND WHATEVER ELSE THEY CAN COME UP WITH. WE WERE LED DOWN THE PRIMROSE PATH BY OUR OWN SHORTCOMINGS; BUT IT CAN AND WILL HAPPEN TO OTHER CREWS OPERATING IN THIS THEATER OF OPS.